Crustal structure and isostatic compensation near the Kane fracture zone from topography and gravity measurements—I. Spectral analysis approach

Summary. Six gravity and bathymetry profiles perpendicular to the Kane fracture zone, each more than 300 km long, were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages. A spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths. For remaining wavelengths the admittance, the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry, is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate, about 6 km in thickness. After subtracting the gravitational attraction of the bathymetry and its compensation, substantial isostatic anomalies remain. We interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography, except at very long wavelengths. The apparent crustal thickness varies by as much as a factor of 2, but there is no evidence indicating systematic thinning of the crust beneath the fracture zone. Our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface. This indicates that there are two different modes of crustal formation with different gravity and topographic signatures: effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies, and intrusive volcanism or underplating producing gravity anomalies but little topographic relief.     

Maximum-likelihood astrometric geometry calibration of interferometric telescopes: application to the Very Small Array

Interferometers require accurate determination of the array configuration in order to produce reliable observations. A method is presented for finding the maximum-likelihood estimate of the telescope geometry, and of other instrumental parameters, astrometrically from the visibility timelines obtained from observations of celestial calibrator sources. The method copes systematically with complicated and unconventional antenna and array geometries, with electronic bandpasses that are different for each antenna radiometer, and with low signal-to-noise ratios for the calibrators. The technique automatically focuses on the geometry errors that are most significant for astronomical observation. We apply this method to observations made with the Very Small Array and constrain some 450 telescope parameters, such as the antenna positions, effective observing frequencies and correlator amplitudes and phaseshifts; this requires only ∼1 h of CPU time on a typical workstation.     

The 2dF Galaxy Redshift Survey: galaxy luminosity functions per spectral type

We calculate the optical b _J luminosity function (LF) of the 2dF Galaxy Redshift Survey (2dFGRS) for different subsets defined by their spectral properties. These spectrally selected subsets are defined using a new parameter, η , which is a linear combination of the first two projections derived from a Principal Component Analysis. This parameter η identifies the average emission- and absorption-line strength in the galaxy rest frame spectrum, and hence is a useful indicator of the present star formation. We use a total of 75 000 galaxies in our calculations, chosen from a sample of high signal-to-noise ratio, low-redshift galaxies observed before 2001 January. We find that there is a systematic steepening of the faint-end slope ( α ) as one moves from passive  ( α =-0.54)  to active  ( α =-1.50)  star-forming galaxies, and that there is also a corresponding faintening of the rest frame characteristic magnitude   M *-5 log₁₀( h )  (from −19.6 to −19.2). We also show that the Schechter function provides a poor fit to the quiescent (Type 1) LF for very faint galaxies  [ M _{b J}-5 log₁₀( h )  fainter than −16.0], perhaps suggesting the presence of a significant dwarf population. The LFs presented here give a precise confirmation of the trends seen previously in a much smaller preliminary 2dFGRS sample, and in other surveys. We also present a new procedure for determining self-consistent k -corrections, and investigate possible fibre-aperture biases.     

The 2dF Galaxy Redshift Survey: the dependence of galaxy clustering on luminosity and spectral type

We investigate the dependence of galaxy clustering on luminosity and spectral type using the 2dF Galaxy Redshift Survey (2dFGRS). Spectral types are assigned using the principal-component analysis of Madgwick et al. We divide the sample into two broad spectral classes: galaxies with strong emission lines ('late types') and more quiescent galaxies ('early types'). We measure the clustering in real space, free from any distortion of the clustering pattern owing to peculiar velocities, for a series of volume-limited samples. The projected correlation functions of both spectral types are well described by a power law for transverse separations in the range  2<( σ / h ^-1 Mpc)<15  , with a marginally steeper slope for early types than late types. Both early and late types have approximately the same dependence of clustering strength on luminosity, with the clustering amplitude increasing by a factor of ∼2.5 between L * and 4 L *. At all luminosities, however, the correlation function amplitude for the early types is ∼50 per cent higher than that of the late types. These results support the view that luminosity, and not type, is the dominant factor in determining how the clustering strength of the whole galaxy population varies with luminosity.     

Emission-line oscillations in the dwarf nova V2051 Ophiuchi

We have detected coherent oscillations, at multiple frequencies, in the line and continuum emission of the eclipsing dwarf nova V2051 Ophiuchi using the 10-m Keck II telescope. Our own novel data acquisition system allowed us to obtain very fast spectroscopy using a continuous readout of the CCD on the LRIS spectrograph. This is the first time that dwarf nova oscillations have been detected and resolved in the emission lines. The accretion disc is highly asymmetric with a stronger contribution from the blueshifted side of the disc during our observations. The disc extends from close to the white dwarf out to the outer regions of the primary Roche lobe.
Continuum oscillations at 56.12 s and its first harmonic at 28.06 s are most likely to originate on the surface of a spinning white dwarf with the fundamental period corresponding to the spin period. Balmer and helium emission lines oscillate with a period of 29.77 s at a mean amplitude of 1.9 per cent. The line kinematics and the eclipse constraints indicate an origin in the accretion disc at a radius of 12±2 R _wd. The amplitude of the emission-line oscillation modulates (0–4 per cent) at a period of 488 s, corresponding to the Kepler period at R =12 R _wd. This modulation is caused by the beating between the white dwarf spin and the orbital motion in the disc.
The observed emission-line oscillations cannot be explained by a truncated disc as in the intermediate polars. The observations suggest a non-axisymmetric bulge in the disc, orbiting at 12 R _wd, is required. The close correspondence between the location of the oscillations and the circularization radius of the system suggests that stream overflow effects may be of relevance.